You can upgrade Artifacts up to 10 times when they reach a certain level.
Successfully upgrading the Artifact will increase the stats of the existing add-on skill.

Upgrading to a certain point will unlock an additional add-on skill instead of increasing the base skill stats.
The add-on skill can overlap with the base skill. In this case, both skill effects will be applied both times.
If there is already an add-on skill, upgrading will increase it rather than unlocking a new one.